A. Technical Field
The present invention relates generally to optical technology, and more particularly, to optical technology in an input device.
B. Background
Optical technology is used in many contexts, including in optical input devices. There are many different types of input devices, including a mouse, a trackball, and a joystick. There are significant advantages to using optical input devices over mechanical and opto-mechanical input devices. For example, mechanical or opto-mechanical input devices have mechanical components that are more susceptible to breakdown or wear out. Optical devices having only solid state components are less susceptible to such breakdown or wear out. However, one disadvantage of some optical input devices is increased power consumption, caused in part by an inefficient illumination source or system. Illumination requires a precise angle of illumination and a sufficient optical power to create a pattern on a surface (e.g., a table surface) that can then be captured by a photosensor. The pattern is the surface pattern itself illuminated by the beam or the light and shadow of the surface microstructure that is generated by the illumination beam impinging at the appropriate angle. In conventional illumination systems, in order to achieve the desired illumination at the desired angle and the desired optical power, large power consumption is required due to an inefficient illumination system. This power consumption shortens battery life in wireless, optical pointing device systems.
As an example of an optical displacement system, consider an optical mouse. The optical mouse includes a conventional illumination system. Conventional illumination systems consist of a light emitting diode (LED) and a double prism system. The double prism system consists of an entrance surface, a double prism, and three exiting facets approximating a cylindrical concave exit surface. The entrance surface is a plano-convex lens shape linked to the double prism body that collects the LED light and collimates it. The double prism conducts the light beam to a target area on the table surface with the required incidence angle. The cylindrical concave exit surface attempts to spread the light evenly on the target area. An imaging lens creates an image of the lighted area on an optical sensor. The double prism system serves as a light conductor between the LED and the table surface (e.g. a table top or mousepad). Conventional illumination systems require that a total internal reflection (TIR) condition be met. A TIR condition is met when an incidence angle of a light ray, for example, inside a plastic media interfaced with air, is larger than a critical angle resulting in total internal reflection at the transparent material surface and no rays are refracted outside the transparent material. However, rays that do not encounter the entrance surface or rays that do not satisfy the TIR condition within the double prism path are lost. In conventional illumination systems, the LED is mounted on a printed circuit board (PCB) in a horizontal configuration on the component side of the PCB. In this conventional configuration, the LED can be easily soldered to the PCB simultaneously with the other electronic components. Thus, to direct the light to the target surface, the double prism is required to achieve both the vertical distance and the required incidence angle.
Conventional illumination systems, using a double prism system, have a long light path, multiple direction changes, and no way to recover diverging rays, thus, increasing loss and reducing efficiency. Furthermore, as the light source, which includes an LED die and LED optics, size is not a single point, it is not possible to accurately focus all rays coming from the LED. There is a significant amount of loss across this conventional system. Examples of four types of loss are: TIR loss, reflection/refraction loss, transmission loss, and coupling efficiency loss. Coupling efficiency loss is caused by the fact that not all light from the LED can get into the double prism because the alignment of the LED with the entrance surface of the prism cannot be perfect and the surface of the entrance lens of the prism is not large enough to collect all the viewing angle emitted by the LED. Each of many intermediate parts contribute to this misalignment, for example, an LED package, an LED support, the PCB, and a mouse case. Due to the above mentioned limitations, the intensity, the uniformity, and the position of the illumination spot are degraded.
Therefore, there is a need for improving the illumination of an optical input device while improving the image signal power on a photosensor. Accordingly, it is also desirable to provide an optical input device with an efficient illumination source that helps reduce power consumption and increase battery life and illuminate the target area uniformly.